Steerable catheter ablation systems are well know for the identification and treatment of a variety of medical conditions. Catheters of this type have one or more electrodes on the far or distal end of a catheter which is inserted into a blood vessel or similar bodily area and steered toward a body part. Once in position, the electrodes can be used for electrically mapping the body part, or for delivering a therapy to the body part.
Catheter ablation was originally developed to treat various vascular diseases by removing blockages in the arteries, for example, as shown by the catheter ablation systems described in U.S. Pat. Nos. 5,078,717 and 5,125,928. Catheter ablation has also been used for thermal ablation treatments of other body organs, as taught, for example, in U.S. Pat. No. 5,222,938. One area where the use of steerable catheter ablation systems has meet with substantial success is in the field of cardiac electrophysiology, particularly for treatment of abnormal heart rhythms known as cardiac arrhythmias. In treating cardiac arrhythmias, the goal of the catheter ablation procedure is to identify the tissue in the heart that may be causing the arrhythmia, and then selectively destroy or disable that tissue without damaging surrounding tissue and muscle.
Identification of a potential ablation site is often done by a process known as mapping in which a catheter having multiple electrodes is inserted into the heart and the electrical signals of the heart are monitored in order to identify the tissue causing the arrhythmia. Examples of these types of catheter mapping systems are shown in U.S. Pat. Nos. 5,263,493, 5,324,284, 5,327,889, 5,454,370, 5,476,495 and 5,487,385. In each of these systems, a plurality of separate electrodes are mounted on one or more flexible members that are then positioned against the interior of a chamber of the heart. In this way, the discrete electrodes each serve as individual antennas for detecting the electrical activity of the heart in the area corresponding to that electrode.
Once an ablation site is identified, a lesion is created that permanently destroys the ability of the cells to propagate the electrical signals of the heart. Early cardiac catheter ablation systems, such as shown in U.S. Pat. No. 4,869,248, utilized a DC current directed through a thin loop of wire at the end of the catheter to burn a lesion around the arrhythmia-producing tissue. Unfortunately, the use of a DC current to heat the wire generated excess heat which caused undesirable coagulation of the blood at the site of the ablation. Another problem was that the wire loop was relatively imprecise, especially for treating small amounts of tissue.
To overcome these problems, most current cardiac catheter ablation systems use a more controlled type of energy source, such as radio frequency (RF) energy, to heat the tissue above 55.degree. C., thereby creating the desired lesion. The more controlled RF energy is also delivered in a more precise manner by using a point source of ablation energy. Examples RF catheter ablation systems which deliver the RF energy as a point source using a needle-like or tip electrode fixed to the distal end of the catheter are shown in U.S. Pat. Nos. 5,083,565, 5,281,218 and 5,403,311. Another solution to the problem of excess heat generation has been to use a coolant to control the temperature of the ablation electrode. Examples of fluid cooled RF ablation catheter systems are shown in U.S. Pat. Nos. 5,334,193, 5,348,554, 5,423,811, 5,431,649 and 5,462,521.
While existing RF catheter ablation systems are well-suited for use in the ablation of localized arrhythmia producing tissue that can be destroyed by using a point source of ablation energy, recent techniques have been developed which require that the ablation energy be delivered as something other than a point source. Once such procedure is the Maze and Corridor procedure developed by Dr. Cox et al. of Washington School of Medicine in St. Louis, Mo. As originally developed, this procedure treats atrial fibrillation or flutter by slicing the atrial tissue in a maze-like pattern in order to prevent the conduction of fibrillation electrical activity through the atrium. It has been discovered that this procedure can also be accomplished by ablation, however, the time and effort required to ablate the extensive maze-like pattern required by this procedure using only a point source of ablation energy makes it generally impractical to attempt this procedure with existing tip electrode RF ablation catheter systems because of the difficulty in dragging a tip electrode across tissue to create a linear lesion.
U.S. Pat. No. 5,487,385 describes an RF atrial mapping and ablation catheter system which is particularity adapted to create the linear lesions necessary to perform procedures such as the Maze and Corridor procedure. In this patent, a flexible working catheter section is provided with a plurality of spaced-apart ring electrodes that are used for both sensing and ablation. The working catheter section is extendable from a main catheter sheath and may be controlled to generate an adjustable arcuate curve that can be positioned against an interior wall of the heart to perform both sensing and ablation. While the use of the same set of electrodes for both mapping and ablation has the advantage of requiring only a single catheter to perform both steps of the procedure, it has a disadvantage in that the nature of the electrodes must be a compromise between a pure sensing electrode and a pure ablation electrode. This comprise, in turn can affect the quality and consistency of the linear lesions which are created using this system. The use of multiple spaced-apart electrodes also makes it necessary to increase the power utilized by the system in order to ensure that the lesions which are created by each individual electrode will be large enough to overlap so as to create the desired linear lesion. This not only causes potential problems with respect to destroying more heart tissue than may be necessary to accomplish the procedure, but also increases the possibility of undesired coagulation of blood at the site due to the higher temperatures created by the increased energy usage. In addition, the existence of multiple electrodes increases the possibility of both mechanical and electrical failure within the system.
Although RF catheter ablation systems have wide application in a variety of medical procedures, most RF catheter ablation systems are not suited for use in creating linear lesions. Accordingly, it would be desirable to provide for an RF catheter ablation system which is simple and reliable, yet is capable of generating high quality, consistent linear lesions.